Veigar is a supreme antimage mage, and since there's a 95% chance that you're going to face a mage, he's a good mid (not to mention that nobody complains about him).

Tips for mid? Sure.

1.) Learn to dodge, ap mids are LOADED with skillshot harass, and this mostly takes place all the time during mid.

2.) Gank side lanes when you are winning your lane.

But most importantly, learn to dodge skillshots or you're not going to get anywhere. Also memorize champ skill ranges.

Actually Jungle is best role to carry with just to let you know. learn how to jungle real well and you will be at plat or diamond in no time. Mid however is second best.

As for 'viable' mids well all are viable just really have to find a condensed few that match your game play and style. So, AD caster mids, Tanky ap mids, Bursty Mages, or something like that. Personally, I love playing Jayce mid, but my ap choices would be anivia learn Anvivia and you can make BIG plays. Ryze is good too. Khazix is a very strong pick now a days but is banned out a lot.